Make the Mayo: In a small bowl, stir together the mayonnaise, miso, and a squeeze of lime juice until smooth. Check the seasonings. The Magic Miso-Mayo will keep, covered in the refrigerator, for up to 2 weeks.
Make the Sliders: Put a gallon-size resealable plastic bag in a medium bowl to hold it steady. Add the pineapple juice, garlic, and pork. Seal the bag, pressing out any excess air, and turn it over a few times to coat the tenderloin. Marinate in the refrigerator for 8 to 12 hours (or at room temperature for up to 1 hour), turning the bag over once or twice, if possible.
Preheat the oven to 400°F, with a rack in the middle position. In a medium ovenproof skillet, heat the oil over medium-high heat until it shimmers. Remove the tenderloin from the marinade, pat dry, then season with salt and pepper. Add the tenderloin to the pan and cook until golden brown all over, about 6 minutes total.
Off the heat, brush the top half of the tenderloin with half the hoisin sauce. Put the pan in the oven and roast for 8 minutes. Turn the tenderloin over, brush the top half with the remaining hoisin sauce, and roast until almost cooked through, but still pink, about 6 minutes more. Transfer the tenderloin to a cutting board and let rest for 5 to 10 minutes.
Cut the pork into 1/4-inch-thick slices. Serve with the rolls, extra hoisin sauce, scallions, Magic Miso-Mayo, and Sriracha, and let everyone assemble their own sliders.